1. Field of the Invention
The present invention relates to an image pickup apparatus, a radiation image pickup apparatus, more particularly an image pickup apparatus and a radiation image pickup apparatus that are capable of reading a signal from a flat panel detector as digital output using a plurality of analog-to-digital (A/D).
2. Description of the Related Art
In recent years, a radiation image pickup apparatus employing a flat panel detector (hereinafter abbreviated as FPD) made of a semiconductor material has been becoming practical as an imaging apparatus for use in medical image diagnosis and nondestructive inspection using X rays. Such a radiation image pickup apparatus employing an FPD is an apparatus that can perform digital imaging of converting radiation, for example, an X ray that transmitted through an object (e.g., patient) into an analog electric signal using the FPD and converting the analog electric signal into a digital image signal. FPDs used in such a radiation image pickup apparatus can be broadly classified into the direct conversion type and the indirect conversion type. A direct-conversion radiation image pickup apparatus is an apparatus that includes an FPD having a plurality of pixels two-dimensionally arranged, each pixel including a conversion element made of a semiconductor material that can directly convert radiation into an electrical charge (e.g., a-Se). An indirect-conversion radiation image pickup apparatus is an apparatus that includes an FPD having a plurality of pixels two-dimensionally arranged, each pixel including a conversion element that has a wavelength conversion member that can convert radiation into light (e.g., fluorescent member) and that has a photoelectric conversion element made of a semiconductor material that can convert light into an electrical charge (e.g., a-Si). Such radiation image pickup apparatuses employing FPDs are used as digital image pickup apparatuses capable of radiographic imaging, like still imaging, and movie imaging, like fluoroscopy imaging.
In addition to an image signal in image capturing based on radiation that transmitted through an object, a correction image signal for use in correction of the image signal in image capturing, such as a correction image signal for use in offset correction or a correction image signal for use in sensitivity correction, is obtained from a radiation image pickup apparatus. A correction signal for use in offset correction is based on output obtained in a state where no light or radiation is applied to a detector (dark output). A correction signal for use in sensitivity correction is based on output obtained in a state where light or radiation is applied to a detector in a substantially uniform manner (bright output). Correcting an image signal in image capturing using such a correction image signal enables the image signal to have low noise and high precision.
The radiation image pickup apparatus includes the above-described detector, a driving circuit for driving the detector, a readout circuit for reading an analog electric signal from the detector, and an A/D converter for converting the analog electric signal into a digital signal. The A/D converter outputs a digitized image signal in image capturing and a correction image signal. To output an image signal in a shorter time from the image pickup apparatus, the image pickup apparatus includes a plurality of A/D converters.
However, a conversion characteristic between an input analog electric signal and an output digital signal in an A/D converter (A/D conversion characteristic) may have nonlinearity that does not exhibit ideal linearity. In particular, for an image pickup apparatus including a plurality of A/D converters, the A/D converters may have different nonlinearities, and an image formed from a digital signal may have artifact, such as a step. In such a case, where artifact, such as a step, occurs, the nonlinearities should be reduced or effects caused by nonlinearities should be compensated for.
Japanese Patent Laid-Open No. 2005-210480 discloses an A/D converter circuit that stores, at an address specified by an output from an A/D conversion unit, a reference signal synchronized using an output signal from the A/D conversion unit as address data and that corrects the output signal from the A/D conversion unit in accordance with the reference signal. The patent document discloses the capability to reduce artifact in image caused by nonlinearity of the A/D conversion unit and to achieve high image quality by use of the above configuration.
Japanese Patent Laid-Open No. 2005-210396 discloses an A/D converter circuit that includes a correcting unit configured to correct output signals from a plurality of A/D conversion units in accordance with an output signal from either one of the plurality of A/D conversion units. The patent document discloses the capability to reduce artifact in image caused by difference between nonlinearities of the plurality of A/D conversion units and to achieve high image quality by use of the above configuration.
The techniques described in the above patent documents can reduce artifact resulting from nonlinearity of an A/D converter by correcting digital output from the A/D converter, as described above. However, there may be an issue in that, for correction described in the above patent documents, a significantly large circuit for correcting digital output is used. There may be also an issue in that the step of previously obtaining conversion data for use in correcting nonlinearity of each A/D converter and the step of performing digital correction for every digital output results in complication of the system.